A. Field of the Invention
The present invention relates generally to rotatable valves, and more particularly to plastic lined rotatable valves adapted for use in handling corresive materials.
B. Description of the Prior Art
Fluid handling systems necessarily include valves. Systems for use in handling corrosive materials or products that must be maintained in a high state of purity necessarily include elements, including valves, having inert internal surfaces. Some valves and systems are made of stainless steel or other corrosion resistant metalic material. Additionally, there have been developed plastic lined valves in which the internal surfaces of the valve body and the valving element have been coated with a plastic material. Fluorinated hydrocarbons such as perfluoroalkoxy resin, which is also known as PFA, have been used successfully in a large number of valves.
Rotatable valves, such as ball valves and plug valves, include a valve body that is split into at least two parts so that the ball or plug may be inserted into the valve body. The valve body parts typically include flanges that are connected together to assemble the valve body by studs or bolts. The seals between the valve body parts are provided by flanges of the plastic liner material which seal directly upon each other. The liner materials, being plastic, tend to flow under compression. Thus, the body seals of heretofore existing plastic lined valves have tended to leak after time due to flow of the material. Also, the plastic lining-to-plastic lining seal of heretofore existing plastic lined valves has tended to leak when piping loads act unevenly on the valve body. For example, if a bending load is transmitted across the valve body, the plastic liner seals on the inside of the bend are compressed and those on the outside of the bend are relaxed, thereby allowing fluid to leak.
The solution to the cold flow problem has been to provide space in which the body parts may be retightened. However, that solution is not entirely satisfactory after corrosive and possibly dangerous materials have leaked out of the valve body. There has been no satisfactory solution to the problem of leaks induced by uneven piping loads across the valve body.
Additionally, the other seals within plastic valves, including the valve seats, must also be of a corrosio resistant or substantially inert material. Typically, such seals are formed from polytetrafluoroethylene, which is better known as PTFE. The plastic seats also tend to flow and over time lose their effectiveness.
It is therefore an object of the present invention to provide a plastic lined rotatable valve that overcomes the shortcomings of the prior art. More particularly, it is an object of the present invention to provide a plastic lined valve that is not subject to leaks due to flow of the plastic liner material. It is a further object of the present invention to provide a plastic lined valve that includes means for compensating for the flow of the plastic material in the valve. It is a further object of the present invention to provide a plastic lined valve that is not subject to leak when the valve body is subjected to uneven piping loads.